Numerous modern electronic devices from the simple wrist watch to the more sophisticated computer servers depend upon the generation of one or more clock or oscillator signals. To meet the needs of various applications, the signals generated should be accurate and stable. In addition, the operational frequencies of the generated signals should not significantly deviate with changes in temperature from the design frequency.
Essentially all cell phones, computers, microwave ovens, and numerous other electronic products use a quartz crystal resonator to generate a reference signal at a pre-selected frequency which is typically around 20 MHz. The gates in these crystal-controlled oscillators are “clocked” or switched at the pre-selected frequency using the reference signal. Any and all “time references” are generated from this quartz resonator-oscillator. Depending upon the specific application, the oscillator may need to have a frequency drift over the product's full operational temperature range of approximately +/−2 ppm. If this level of frequency control is necessary, the quartz resonator may need to be fabricated in an expensive hermetic ceramic package with a metal lid that is arc-welded around the perimeter. In applications such as cell phones, laptop computers, and other portable devices, this type of package may be larger than desirable. Also, as this crystal's resonant frequency is often approximately 20 MHz, the signal from the oscillator must often be multiplied upwards by other power consuming electronics with resultant harmonics generally suppressed by only approximately 5 dB relative to the fundamental frequency.
Oscillators can also be constructed using other types of resonators such as standard L-C (inductor-capacitive) resonators, thin film bulk acoustic resonators (FBARs), and the like. While such resonators are less expensive than quartz resonators their frequency drift characteristics are generally less than acceptable for the applications mentioned above, as well as for others. In particular, thin film bulk acoustic resonator devices are designed to resonate at a specific frequency which in one design is 1900 MHz. But, over a typical operating temperature range of −30° C. to +85° C., their resonating frequency can vary several MHz due to the frequency variation with temperature of the constituent materials. If the allowable frequency band for the device is narrow, the frequency shift with temperature may be so great as to shift the operating frequency of the device beyond the allowable frequency band.
To alleviate the problem of drift with temperature in FBARs, work has been done in developing compensation layers to reduce or eliminate the shift of resonant frequency with temperature. Several recent studies have shown the effectiveness of silicon dioxide layers deposited at various locations within the stack structure.
However, current FBAR processes utilize a sacrificial layer of phosphosilicate glass (8% phosphorus, 92% silicon dioxide). Subsequent layers of the device are deposited, sequentially, upon the phosphosilicate glass until the final structure is developed whereupon the phosphosilicate glass is etched away with hydrofluoric acid leaving a free-standing structure. Unfortunately, silicon dioxide is etched rapidly in hydrofluoric acid making silicon dioxide unsuitable as a temperature compensation layer with current FBAR fabrication processes.